The present invention relates to a vehicular skid control system and particularly to an improvement in a switch device which generates an electric signal when a vehicular skid control system is in an operating condition for eliminating a skid condition of a vehicle.
As is well known in the art and as is shown in FIG. 1 of the drawings, a vehicular skid control system usually includes a body having an inlet port 1 connected to a master cylinder MC, an outlet port 2 connected to a rear wheel brake cylinder WC of a vehicle, and passages 3 and 4 communicating respectively with the inlet and outlet ports 1 and 2. A check valve 5 is disposed between the passages 3 and 4 to control fluid flow from the passage 3 to the passage 4. During normal braking operation, the check valve 5 is opened by an actuator or expansion piston 6 which is moved in a direction by pressurized hydraulic fluid fed from a pressurized hydraulic fluid source P to a working chamber. When the vehicle is in a skid condition, a valve stops the supply of pressurized hydraulic fluid to the working chamber and connects the working chamber to a sump D to exhaust hydraulic fluid in the working chamber. As a result, the expansion piston 6 is moved in a reverse direction to cause closing of the check valve 5 and concurrently to reduce the pressure of hydraulic fluid confined in the passage 4 and a passage 7 between the outlet port 2 and the rear wheel cylinder WC to remove the skid condition of the vehicle. A switch-over valve 8 is disposed between the outlet port 2 and the check valve 5 and normally allows communication between the passage 4 and the outlet port 2. In the event of failure of the presurized hydraulic fluid source P, (viz., a loss of hydraulic pressure) the switch-over valve 8 directly connects the inlet port 1 to the outlet port 2 by way of a bypass passage 11 without passing the passages 3 and 4 to insure the normal braking operation only. The skid control system usually also includes a switch device which senses a skid control operation of the system for providing an indication thereof to a driver.
In the prior art, the switch device 9 includes, as shown in FIG. 1, a switch piston 10 having one end surface 10a communicating with the passage 4 to receive a rear wheel braking fluid pressure, and the other end surface 10b communicating with the passage 3 to receive a master cylinder fluid pressure, an electric conductor 14 fixed to the switch piston 10, and a switch taking the form of brushes 12 and 13 electrically connectable to and disconnectable from each other by the electric conductor 14. During normal braking operation in which the fluid pressures in the passages 3 and 4 are equal to each other, the electric conductor 14 is moved rightwards in the drawing by the hydraulic pressure acting on the end surface 10a larger in area than the end surface 10b to electrically disconnect the brushes 12 and 13 from each other. On the other hand, during a skid control operation in which the rear wheel braking fluid pressure is extremely lower than the master cylinder fluid pressure, the electric conductor 14 is moved by the fluid pressure acting on the end surface 10b leftwards in the drawing to electrically connect the brushes 12 and 13 to each other. In response to closing of the switch, an indicator 15 is actuated by an indicator circuit 16 to indicate that the skid control system is under the skid control operation.
The end surface 10a of the switch piston 10 communicates with the passage 4 at a portion upstream of the switch-over valve 8. This upstream portion is separated from the passage 3 by the check valve 5 and from the passages 7 and 17 by the switch-over valve 8 in the event of failure of the source P. Thus, at this time, the hydraulic fluid pressure acting on the end surface 10a is reduced so that the switch is closed to indicate that the system is under a skid control operation in spite of the fact that it is not actually so. For solving this problem, it is considered feasible, for example, to indicate in combination a signal representative of failure of the source P or operation of the switch-over valve and a signal representative of a skid control operation. However, this solution suffers from the drawbacks that the number of component elements is increased and the hydraulic fluid circuit is complicated.
Also, in the conventional skid control system, since the switch piston 10 is moved during a skid control operation as mentioned above, it is necessary to replenish pressurized hydraulic fluid corresponding to the displacement of the switch piston 10 from the master cylinder. Since the replenished hydraulic fluid is returned to the master cylinder when the skid control operation is finished, the system suffers from the drawback that a shock is given to a brake pedal.
Furthermore, in the conventional skid control system, since the switch piston 10 operating the electric conductor 14 is located in hydraulic fluid, it is necessary to provide a seal between the switch piston 10 and the electric conductor 14. Thus, the system suffers from the drawback that a malfunction of the switch device 9 occurs when a seal is defective.